2007ApJ...667..131G

We present the first measurement of the BH mass function for broad-line active galaxies in the local universe. Using the ∼8500 broad-line active galaxies from SDSS DR4, we construct a broad-line luminosity function that agrees very well with the local soft X-ray luminosity function. Using standard virial relations, we then convert observed broad-line luminosities and widths into BH masses. A mass function constructed in this way has the unique capability to probe the mass region <10⁶ M_☉, which, while insignificant in terms of total BH mass density, nevertheless may place important constraints on the mass distribution of seed BHs in the early universe. The characteristic local active BH has a mass of ∼10⁷ M_☉ radiating at 10% of the Eddington rate. The active fraction is a strong function of BH mass; at both higher and lower masses the active mass function falls more steeply than one would infer from the distribution of bulge luminosity. The deficit of local massive radiating BHs is a well-known phenomenon, while we present the first robust measurement of a decline in the space density of active BHs at low mass.